The invention relates to a configuration for multiplexing and/or demultiplexing the signals of at least two optical wavelength channels. The configuration contains an optical grating device for transforming a first optical power of each of the channels. The optical grating device has a specific common region of space assigned in common to all of the channels to which the first optical power is fed and is transformed into a second optical power that is concentrated in each case to one of a plurality of specific separate regions of space each assigned to one of the channels alone, and vice versa. The optical grating device contains an optical grating, a first optical free beam region disposed between the specific common region of space assigned in common to all of the channels and the optical grating, a second optical free beam region disposed between the optical grating and each of the specific separate regions of space are each assigned to one of the channels alone, and a coupling device for coupling light into or out of all of the channels into the first free beam region. The light coupled in or out traverses the specific common region of space assigned in common to all of the channels. A plurality of waveguides is provided. One of the waveguides is assigned to each of the channels alone, the waveguides are coupled in each case to the second free beam region.
A multiplexing/demultiplexing configuration of the generic type is disclosed, for example, in International Patent Disclosure WO 96/00915. The configuration described there has a grating device that is used both to separate and to combine the wavelength channels. The grating device has an optical grating, an optical free beam region that is disposed between a point in space assigned in common to all channels, and the optical grating, and an optical free beam region that is disposed between the optical grating and each point in space assigned one channel alone.
In a particular embodiment of this type, the optical grating contains a phased array, that is to say of a plurality of strip-shaped optical waveguides, each of which has a first end face which faces the point in space assigned in common to all channels, and a second end face which faces the points in space, to each of which one channel each is alone assigned. An optical length between the first end face and the second end face varies from waveguide to waveguide.
If, as a demultiplexer, the particular embodiment is operated in the case of which the channels are spatially separated, the first end faces of the waveguides of the phased array form entrance apertures of the grating, and the second end faces of the waveguides form exit apertures of the grating. When this embodiment is operated as a multiplexer, in the case of which the spatially separated channels are combined, the second end faces of the waveguides of the phased array form entrance apertures of the grating, and the first end faces of the waveguides form exit apertures of the grating. The waveguides of the phased array act in all cases as optical phase gratings.
Instead of a grating in the form of a phased array, it is also possible to use other optical gratings, for example etched gratings (see IEEE, Photonics Technology Lett., Vol. 8, No. Oct. 10, 1996, pages 1340 to 1342).
The grating device of such a configuration determines a wavelength-dependent transmission function of each strip-shaped optical waveguide that is assigned to one channel alone and/or to all channels in common, and an end face that faces the grating device and is disposed at the point in space that is assigned to one channel alone or all channels in common. The transmission function is a Gaussian function, at least to a first approximation (see the above-mentioned IEEE document).
A more rectangular profile of the wavelength-dependent transmission function of such a waveguide would be more favorable such that in the event of fluctuations in the ambient temperature and/or wavelength the insertion loss of the waveguide changes only insignificantly in a certain wavelength region.
Various possibilities have been described for flattening the inherent quasi-Gaussian transmission function of such a waveguide, that is to say for shaping it in a more rectangular fashion.
Thus, it is known from Electr. Lett., 30, 1994, pages 300-301 to configure the waveguide assigned to one channel alone not, as customary, as a monomode waveguide, but as a multimode waveguide in order to flatten the transmission function thereof.
It is known from Proc. ECOC, Birmingham 1997, Conference Publication No. 448, IEEE 1997, pages 79, 82 to interleave two slightly different phased arrays with one another such that in the case of the point in space of the configuration that is assigned to one channel alone two quasi-Gaussian transmission functions specifically displaced spectrally are superimposed on one another to form a broader flattened transmission function.
It is also known to configure a configuration such that in the case of the point in space of the configuration that is assigned to all the channels in common two mutually overlapping quasi-Gaussian transmission functions are present that can be implemented with the aid of a 3-dB beam splitter (see U.S. Pat. No. 5,412,744), with the aid of what is termed a multimode interference coupler (see above-mentioned IEEE document) and/or with the aid of what is termed a horn structure (see Electr. Lett. 32, 1996, pages 1661-1662). The flattened transmission function, produced at this point in space, in the form of the two overlapping quasi-Gaussian transmission functions is projected by the grating device onto each point in space of the configuration that is assigned to one channel alone.
In the case of the three last mentioned implementations, the decisive flattening operation is the formation of a convolution integral from an electric field distribution in accordance with the overlapping quasi-Gaussian transmission functions with the Gaussian-type mode of each waveguide assigned to one channel alone.
It is known from Optics Lett. 20, 1995, pages 43-45 to change the electric field distribution in the case of the second end faces, forming the exit apertures of the grating, of the waveguides of the phased array. The basis of this implementation is that the free beam region disposed between the end faces and the separate points in space each assigned to one channel alone has a lens effect, and the electric field distribution at the end faces and the electric field distribution at the separate points in space are therefore linked via a Fourier transformation. Through suitable selection of the cross section of the waveguides of the phased array and an additional change to the optical length of the waveguides, it is possible to produce an electric field distribution with, correspondingly, the sin(x)/x function at the second end faces of the waveguides, which function is transformed by the Fourier transformation into a rectangular field distribution at a separate point in space.
Finally, International Patent Disclosure WO 99/52003 describes a configuration for spatial separation and/or combination of at least two optical wavelength channels having an optical phased array device that has a device for generating an attenuation function for a wavelength-dependent attenuation of the transmission function of at least one of the waveguides. Consequently, a suitable field distribution is subtracted from the field distribution of the uninfluenced phased array. This cuts off the top part of the transmission curve.
It is accordingly an object of the invention to provide a configuration for multiplexing and/or demultiplexing the signals of at least two optical wavelength channels which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which configuration provides a rectangular or flatter profile of the filter curves for the individual wavelengths.
With the foregoing and other objects in view there is provided, in accordance with the invention, a configuration for multiplexing and/or demultiplexing signals of at least two optical wavelength channels. The configuration contains an optical grating device for transforming a first optical power of each of the channels. The optical grating device has a plurality of specific separate regions of space and a specific common region of space assigned in common to all of the channels to which the first optical power is fed and transformed into a second optical power that is concentrated in each case to one of the specific separate regions of space each assigned to one of the channels alone, and/or vice versa. The optical grating device contains an optical grating, a first optical free beam region disposed between the specific common region of space assigned in common to all of the channels and the optical grating, and a second optical free beam region disposed between the optical grating and each of the specific separate regions of space each assigned to one of the channels alone. A coupling device is provided for coupling light into or out of all of the channels into the first free beam region, the light coupled in or out traversing the specific common region of space assigned in common to all of the channels. The coupling device for coupling the light into and out of the first optical free beam region effecting at least one of a flattening and a widening of a field distribution of the light coupled in or out at least in a plane of the first free beam region. A plurality of waveguides is provided. One of the waveguides is assigned to each of the channels alone, the waveguides are coupled in each case to the second free beam region.
Consequently, the solution according to the invention is distinguished in that the coupling device for coupling light into or out of all the channels in the first free beam region effects a flattening and/or widening of the field distribution of the light coupled in or out, at least in the plane of the free beam region. A flattened and/or widened field distribution (input field distribution) is therefore present in the vicinity of the region in space assigned to all channels.
Since the configuration is of a mirror-image type with respect to the first and second free beam regions, a widened input field distribution in the first free beam region is projected onto the output of the second free beam region at which each wavelength channel is assigned a dedicated region in space. Consequently, a field flattening or field widening is also present in the regions in space of the second free beam region, which are assigned in each case to one channel alone.
Consequently, the filter curve for the individual wavelengths respectively exhibits a rectangular or flatter profile. It follows that in the event of fluctuations in the ambient temperature and/or the wavelength of the laser radiation the insertion loss changes only insignificantly in a relatively large wavelength region.
The invention is based on the idea of undertaking to widen the filter curves of the multiplexer/demultiplexer configuration not measures at the actual multiplexer/demultiplexer, but by measures that vary or widen the input field distribution (or output field distribution). As a result, there is no need to change the geometry and configuration of the actual multiplexer/demultiplexer. The input signal, of enlarged or flattened bandwidth, is projected onto the output symmetrically by the multiplexer/demultiplexer.
In a preferred refinement of the invention, the coupling device contains a waveguide that has an increased core diameter in the region of the coupling onto the first free beam region. Therefore, a mode field diameter of the waveguide is widened for the light coupled in or out in comparison with the mode field diameter of a monomode fiber, and this leads to a widened spatial input field distribution. Consequently, the filter curve of the individual wavelength is also widened.
The waveguide preferably has a continuous enlargement of the core diameter in the region of the coupling onto the first free beam region (tapered fiber). The core diameter is widened by, for example, heating and/or compressing the monomode waveguide and/or causing it to swell.
In an alternative refinement, the coupling device has a waveguide with two parallel waveguide cores spaced apart from one another that are both coupled to the first free beam region. The Gaussian functions of the two waveguide cores are slightly displaced spatially relative to one another in this case and are superimposed on one another to form a widened Gaussian function as an input field distribution that is projected onto the output of the second free beam region by the optical grating device. The two cores in this case preferably have a spacing of 10 to 20 xcexcm.
In a further refinement of the invention, the coupling device has a waveguide with an elliptical core that likewise leads to a widening of the input field distribution in the first free beam region.
In a further variant of the invention, the coupling device have a Y-coupler, whose single end is coupled to a waveguide, and whose two parallel ends are coupled to the first free beam region. The two parallel ends once again produce in this case two Gaussian functions that are slightly displaced spatially relative to one another and are superimposed on one another to form a widened input field distribution.
The first free beam region of the configuration is preferably disposed at a substrate edge. The coupling device contains in this case a glass fiber that is coupled to the substrate edge in a way known per se. Since a change in the input field distribution of the first free beam region is ascribed solely to measures that are performed in the device for coupling light in and out (optical coupling device), there is advantageously no need to change or adapt the optical grating device formed on the substrate.
The waveguide of the optical coupling device is preferably a monomode glass fiber, since monomode glass fibers transmit the most optical power. In particular, in the optical grating device of the multiplexer/demultiplexer higher modes are radiated in any case, and thus only the fundamental mode is transmitted.
In accordance with a concomitant feature of the invention, the optical grating contains a plurality of strip-shaped optical waveguides. The strip-shaped optical waveguides have first end faces facing the specific common region of space assigned in common to all the channels and second end faces each facing one of the specific separate regions of space to each of which one of the channels is alone assigned. An optical length between each of the first end faces and each of the second end faces varies between the strip-shaped optical waveguides.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a configuration for multiplexing and/or demultiplexing the signals of at least two optical wavelength channels, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.